Four-wheel drive motor vehicles are gaining popularity because of improved driving characteristics and safety and the possibility for the driver to drive more actively.
Such a motor vehicle is often based on a “normal” two-wheel drive design, either with front wheel drive or rear wheel drive. The driven axle may be called a primary drive axle.
The powertrain of the vehicle is supplemented with means transmitting engine power also to the other axle, called the secondary drive axle. A so called limited slip coupling is arranged in these power transmitting means to the secondary axle. The function of such a coupling is to reduce the difference in rotational speed between its ingoing shaft connected to the primary drive axle and its outgoing shaft connected to the secondary drive axle. This means that the engine power is distributed between the two vehicle axles depending on road and driving conditions.
A good example of such a limited slip coupling is shown in EP-B-0 837 998, to which reference is made. This coupling is a hydraulic, lamella type coupling and is electronically controlled, which means that the characteristics of the coupling may be very accurately controlled (together with other systems in the motor vehicle) for obtaining desired conditions.
Couplings with other designs may be used in four-wheel drive motor vehicles.
However, four-wheel drive motor vehicles with such a limited slip coupling may under certain conditions have a weakness in its driving characteristics (even with an electronically controlled coupling). The weakness stems from the fact that the secondary drive axle cannot rotate faster than the primary drive axle, but at most quite as fast (which occurs when the coupling is locked). The reasoning is based on a ratio in the coupling of 1:1.
This weakness or limitation leads to the following results: